Some encoders, to handle more than one active input signal, require extra circuits to prevent more than one encoder input from being active. These additional circuits not only increase the number of required circuits but also add undesirable delays to the circuit.
Other encoders, known as prioritized encoders, are designed such that, when more than one input is active, a code is provided which identifies one of the active inputs as being valid with no indication that any other of the inputs may also be active.
Still other encoders use the "ripple inhibit" approach in which the inputs are numerically ordered and each input, beginning with input one, is evaluated and if any one input is activated all the other higher ordered inputs are blocked and prevented from affecting the output code. This effect is propagated all the way to the penultimate input which if active blocks the ultimate input. For any significant number of inputs, this ripple approach, although quite useful, and even when provided with carry look-ahead is very time consuming and requires significantly more circuitry.
Accordingly, there now exists a need for an improved encoder network which avoids all the above described problems associated with encoder networks found in the prior art.
The present invention, achieves these desirable results, by applying the valid output of a first two bit encoder as a data input to a later stage of encoding which, when combined with a two way selector driven by a valid output of a the first stage encoder for selection of highest match, reduces the number of required circuits and their attendant delay. The present invention thus significantly reduces the logic propagation time usually encountered with such networks.